Low pressure gas filled thermionic valve



June 21, 1960 K. G. cooK ETAL. 2,942,136

LOW PRESSURE GAS FILLED THERMIONIC ,VALVE Filed Aug. 9, 1956 2Sheets-Sheet 1 INVE N'I'OR5 G a- 614w 6 RTTORNEYS June 21, 1 960 K. G.cooK ETAL LOW PRESSURE GAS FILLED THERE/[IONIC VALVE Filed Aug. 9, 19562 Sheets-Sheet 2 15 INVENTDRS RTTdRuEYS United States atent LOW PRESSUREGAS FILLED THERMIONIC VALVE Kenneth George Cook, Northwood, and GeorgeGlynn Isaa'cs, Harrow, England, assignors to The M-O Valve CompanyLimited, London, England Filed Aug. 9, 1956, Ser. No. 602,977 1Claims'priority, application Great Britain Aug. 11, 1955 6 Claims. 01.313-186) This invention relates to low pressure thyratrons.

In such valves it is often desirable that the maximum hold-off voltageshould be as high as possible consistent with-the use of a relativelyhigh pressure for the gas filling. By maximum hold-oh? voltage is meantthe maximum potential difference which may be applied between the anodeand cathode of the thyratron which makes the anode more positive withrespect to the cathode without causing a discharge to occur. The valueof the hold-off voltage is limited by the occurrence of spark ibreakdownbetween the anode and another electrode. The occurrence of thisbreakdown is governed by two .ffactors, Paschens law and electronicfield emission. -Paschen3s law states that the voltage at whichbreakdown coccursbetweentwo electrodes is a function of the product M ofthe pressure of the gas between the electrodes andthelengthoftheelectric lines of force between the electrodes. .Thisfunction passes through a minimum cvalue at :a particular value. of theproduct M; the present invention is concerned only withthyratrons having.low ipressure gas ifillingsythat is .to say, those in which dis-.chargesoccur under conditions such that the value of rthe product Misllower than that corresponding to the minimum value of the breakdownvoltage referred to .above. (It is to .bezunderstood that in thisspecification the term gas includes a vapour.) It will be apparent :thatin suchthyratrons, to obtain high hold-off voltages .for a given gaspressure, the electric lines of 'force should be as short as possiblebetweenthe electrodes between whichbreakdown may occur.

.It is known in a thyratron to surround the anode by the controlelectrode, the cathode being situated outside the latter, so as torestrict effectively the length of the lines of force beween the anodeand the control electrode and hence raise the maximum hold-ofi voltagefor a given "-gaspressure. ByJeducing the. electrode spacing the linesof. force can be shortened, but should the spacing become too closeelectroniefieldsemission will occur, independ- -:en'tly of the value .ofthegas pressure, .and spark breakdown will take place. The requirementsof high maximum hold-01f voltage and relatively high gas pressure aretherefore to some extent mutually incompatible.

It is an object of this invention to provide a low pressure thyratron inwhich an improved balance between these two requirements may beobtained.

According to this invention there is provided a low pressure thyratronin which the anode is disposed within and surrounded by the controlelectrode, the cathode being situated outside the control electrode andat least one further electrode being interposed between the anode andthe control electrode so as to surround the anode.

It will be understood that the extent to which the electrodes surroundthe anode is limited by the requirement that the discharge must passfrom the cathode to the anode and by the need for leads to pass from thevarious electrodes out of the thyratron.

According to a feature of this invention the thyratron is associatedwith means whereby potentials may be apice plied to the anode, thefurther electrode or electrodes and the control electrode in such amanner "that the potential or potentials of said further electrode orelectrodes is or are intermediate between the potentials of the anodeand control electrode, the potential differences across the consecutivedischarge gaps between the anode and the control electrode beingsubstantially equal.

One arrangement in accordance with the invention will now be describedby way of example with reference to the accompanying drawings in which:

Figure 1 is a longitudinal sectional representation of a. hydrogenfilled thyratron; and

Figure 2 is an enlarged sectional representation of part: of thethyratron.

Referring to Figure 1, the thyratron has a sealed glass envelope 1,filled with hydrogen, in which is mounted an electrode structureincluding a thermionic cathode generally designated 2 partiallysurrounded by a heat, shield 3, a control electrode generally designated4, an: auxiliary anode generally designated 5, and a main anode: 6,these electrodes being respectively provided with loads .7, 8, 9 and 1t)sealed through the envelope 1. The cathode 2 has a cylindrical emissiveportion 11 and is pro-' vided with a heater 12 having one end connectedto the: cathode 2 and the other end connected to a separate lead 13sealed through the envelope 1.

The main anode 6 consists of a molybdenum disc 4.16 centimetres indiameter disposed perpendicular to the: axis of the cathode 2. To thecentre of the disc 6 is welded the anode lead 10, which is sheathed overthe major portion of its length by a coaxial cylindrical glass: sheath14 of 4 millimetres outer diameter, the sheath 14 uniting with theenvelope 1 in the region where the lead 10 issealed throughthe envelope1.

The main anode 6 is disposed within and surrounded by the auxiliaryanode 5 which consists of a cylindrical nickel box 15 of internaldiameter 4.86 centimetres and internal length 7 millimetres, the planeends 16 and 17 of the box 15 being disposed parallel to the main anode 6and each being spaced 3.3 millimetres from it. The central portion ofthe plane end 17 is perforated,iand a disc-like baffle 18 of 31millimetres diameter is connected to the box 15 spaced 3 millimetresfrom the plane end 17, the bafiie 18 screening substantially completelythe main anode 6 from the control electrode 4 and the cathode 2.

An annular flange 19 is attached to thebox 15 and forms a longitudinalextension to it, theannulus lying in the same plane as the disc 18 andbeing concentric with it and spaced apartfrom it by 5 millimetres.

Surroundingthe auxiliary anode 5 is'the control electrode 4 consistingof a cylindrical nickel box 20 of internal diameter 5.64 centimetres andinternal length 1.82

centimetres, theiboxes :15 and20Jbeing disposed coaxially. The planeends 21 and 22 of the box 21? are respectively parallel to the planeends 16 and 17 of the box 15, the: plane end 21 being spaced 3.5millimetres from the plane; end 16 and the plane end 22 being spaced 3.5millimetres; from the battle 18. The plane end 22 is perforated, and adisc like bafile 23 of 2.975 centimetres diameter is;

connected to the box 20 so as to be spaced 3.5 millimetres; from theplane end 22, the baffle 23 screening substantially completely thecontrol electrode 4 and the bafile: 18 from the cathode 2. A cylindricalnickel sleeve 24 is; attached to the plane end 22 and surrounds part ofthe cathode heat shield 3, the sleeve 24 being held off by means ofinsulating supports 26 and 27 from an annularflange 23 which is rigidlysecured to the heat shield 3; the: control grid lead 8 is connected tothe control electrode 4 via a spring connector 29 one end of which iselec trically connected to the sleeve 24 via a bolt 25 which is.insulated from the flange 28.

Referring now to Figure 2 of the accompanying draw 3 ings, a circularaperture 7.5 millimetres in diameter is formed concentrically in theplane end 16 of the box 15, the main anode lead passing through thecentre of aperture. A cylindrical glass sleeve of inner diameter 6millimetres and 1.9 millimetres thickness'is disposed coaxial with themain anode lead 10 and unites at one end with the envelope 1, the otherend being contiguous with the plane end 16. A cylindrical nickel sleeve31 is welded at one end to the plane end 16 and A coaxial cylindricalglass sleeve 34, of inner diameter 1.45 centimetres and outer diameter1.75 centimetres, units at one end with the envelope 1, the other endbeing coplanar with the outer surface of the plane end 21 of the box 20.A coaxial cylindrical nickel gauze sleeve is contiguous with the innersurface of the glass sleeve 34 and is supported by a short cylindricalnickel sleeve 36 which is welded at one end to the plane end 16.

A circular aperture 2.05 centimetres in diameter is formedconcentrically in the plane end 21 of the box 20'. A coaxial cylindricalglass sleeve 37, of 2.05 centimetres inner diameter and 2.35 centimetresouter diameter, unites at one end with the envelope 1, the other endbeing contiguous with the plane end 21. A coaxial cylindrical nickelgauze sleeve 38 is contiguous with the outer surface of the glass sleeve37, extending from the plane end 21 to the region where the glass sleeve37 unites with the envelope 1. A coaxial longitudinally splitcylindrical nickel sleeve 39 intimately surrounds the gauze sleeve 38and is welded at one end to the plane end 21. A nickel clamping collarsurrounds the coaxial assembly and is provided with a nut and bolt (notshown) whereby the collar 40 clamps together the split sleeve 39, thegauze 38 and the glass sleeve 37, thus ensuring the correct dispositionof the control electrode 4 with respect to the other electrodes.

It will be appreciated that the provision and disposition of the nickelsleeves 31, 3'5, 38 and 39 and the accurate dimensioning of the glasssleeves 30, 34 and 37 reduce the lengths of the lines of force betweenneighbouring electrodes and their leads and thus reduce the possibilityof long path discharge breakdown, as well as ensuring adequateinsulation of all high voltage leads and the avoidance of large gasfilled cavities. It is desirable that the glass used for the sleeves 30,34 and 37 and for the sheath 14 should have a high resistance to thermalshock and a high dielectric strength.

In operation of a thyratron of the kind described above, the anodevoltage applied to the thyratron is distributed 2,942,136 p p I a bymeans of a resistive potentiometer connected between the main anode 6and the cathode 2 and having a tapping connected to the auxiliary anode5, the control electrode 4 beingmaintained substantially at cathodepotential. In this way, whilst the total anode voltage applied to thethyratron may be 50 kilovolts the potential difference between the mainand auxiliary anodes 6 and -5 and between the'auxilia'ryanode 5 and thecontrol electrode 4 is only 25 kilovolts, this potential difference notbeing sufficient to cause breakdown with the electrode spacingsdescribed above when the thyratron is filled with hydrogen at a pressureof 0.5 millimetre of mercury.

We claim:

1. A low pressure thyratron comprising a sealed envelope having a lowpressure gas filling, and an electrode system disposed within, andprovided with leads sealed through, the envelope, said electrode systemincorporating a control electrode, an anode disposed within andthreedimensionally surrounded by the control electrode, a cathodesituated outside the control electrode, and at least one furtherelectrode interposed between the control electrode and the anode, saidfurther electrode being threedimensionally surrounded by the controlelectrode and three-dimensionally surrounding the anode, all of saidfour electrodes being electrically insulated from one another.. t

2. A low pressure thyratron according to claim 1 wherein means isincluded to apply potential to the anode, to each further electrode andto the control electrode in such a manner that the potential of eachfurther electrode is intermediate the potentials of the anode and thecontrol electrode, the potential difierence across consecutive dischargegaps between the anode and the control electrode being substantiallyequal.

3. A low pressure thyratron according to claim 1, in which there areprovided at least one metallic and at least .one non-metallic screen foreach of the leads to the anode which the gas filling is hydrogen at apressure of the order of 0.5 millimeter of mercury and in which thespacings between consecutive electrodes from the control electrode tothe anode are of the order of 3.5 millimeters.

References Cited in the file of this patent. UNITED STATES PATENTS1,874,753 Hull Aug. 30, 1932 2,443,205 Stutsman June 15, 1948 2,492,666Sloan Dec. 27, 1949 2,514,165 Ramsay July 4, 1950

